One of the major concerns of designers of antenna system communication links is the elimination or reduction of external interference sources, such as jamming, self-interference, atmospheric noise, man-made noise and acoustic noise. As is well known, most of the approaches which attempt to resolve these problems of external interference do so in a relatively complex manner, often times utilizing very large directional antennas and/or with antennas having hundreds, or even more, elements. This problem of external interference is particularly prevalent in the area of communication systems where omnidirectional antennas are employed because of the large number of users operating on the same frequency band and because of multi-path. One known approach involves the use of an antenna configuration comprised of an omnidirectional antenna in combination with a notch antenna at the receiving end of a transmission link to cancel interference arriving from all directions except over the narrow notch beamwidth or null formed by the notch antenna where the desired signal emanates. By orthogonally combining the antenna signals from the two antennas, all of the undesired interference is subtracted from the signal received by the omnidirectional antenna, leaving only the desired signal.
One such system is shown and described, for example, in U.S. Pat. No. 4,431,999, entitled, "Interference Cencelling System Using A Notch Antenna And Omnidirectional Antenna", which issued to Frank S. Gutleber, the present inventor, on Feb. 14, 1984. A further illustration of this concept is shown and described in U.S. Pat. No. 4,275,379, entitled, "Interference Cancelling Random Access Discrete Address Multiple Access System", which issued to Frank S. Gutleber on June 23, 1981. The teachings of these patents are specifically incorporated herein by reference.
Accordingly, it is an object of the present invention to provide an improvement in antenna systems having an interference cancelling capability.
It is another object of the invention to provide an improvement in antenna systems having the capability of orthogonally separating and isolating a plurality of simultaneous interference sources and substantially cancelling the signals emanating therefrom.
It is a further object of the invention to provide an antenna system which can cancel or eliminate interference from multiple sources that are offset from antenna boresight and arrive in the sidelobe region or mainlobe of a large high gain antenna.
It is still another object of the invention to provide an antenna system which provides substantial anti-jam protection for transmission links that employ large high gain antennas such as used in a tropospheric, line of sight, or satellite link.
These and other objects are achieved by an approach where adaptive antenna interference cancelling is obtained by the isolation and independent tracking of multiple jammers that are received in a sidelobe region of a primary high gain directive antenna. Each of the interferring signals is orthogonally separated from all of the other interferring signals and then it is directly and separately subtracted from a composite input signal of the desired signal plus all the interference signals received by the high gain antenna. Cancelling the interference does not involve perturbations where the reduction of one interferror can result in increasing the signal of another interferror, nor does it require complex adaptive control algorithms.
The interference cancelling concept of the present invention involves a multiplexed interference cancelling system that utilizes a primary high gain antenna and an auxiliary antenna pair including a low gain notch antenna in conjunction with a low gain omnidirectional antenna. The output of these two relatively simple low gain antenna structures are subtracted from each other to provide an output that is substantially free of all input signals except for the interferring signal received in the narrow notch of the notch antenna. This signal is then subtracted from the composite output signal of the primary antenna. Employing one of these antenna pairs as an auxiliary antenna pair for each separate jamming signal source to be cancelled enables each jamming signal to be orthogonally isolated, whereupon its magnitude and phase is then adjusted to be equal to and 180.degree. out of phase with the same component contained in the composite signal from the primary antenna. Each interfering signal is then separately subtracted from the composite signal, leaving only the desired signal.